01One formula
For sensible heating: P = ṁ × c × ΔT / 3600 (kW), with ṁ in kg/h, c in kJ/(kg·°C), ΔT in °C. With phase change (boiling water to steam, for example), add the latent term: P_latent = ṁ × r / 3600, where r is the latent heat of vaporization in kJ/kg.
For static (batch) heating of a tank, use P = m × c × ΔT / (3600 × t), with t the required heat-up time in hours. Add the heat absorbed by the tank steel and the losses through insulation — on large tanks that alone can be 20–30% of the total.
02Property quick table
Water
c ≈ 4.18 kJ/(kg·°C), ρ ≈ 1000 kg/m³. Rule of thumb: 1 tonne of water per hour per °C ≈ 1.16 kW.
Thermal oil
c ≈ 2.0–2.3 kJ/(kg·°C) (varies strongly with temperature), ρ ≈ 850–950 kg/m³. Half the heat capacity of water per kilogram — which is why oil systems circulate harder.
Air
c ≈ 1.01 kJ/(kg·°C), standard ρ ≈ 1.29 kg/m³. Every 1,000 Nm³/h per °C ≈ 0.36 kW.
Steam (latent)
r ≈ 2,257 kJ/kg at atmospheric pressure. One tonne per hour of steam ≈ 630 kW of pure latent duty.
03Three loads people forget
Surface losses
Radiation and convection from uninsulated or outdoor equipment: typically 5–15% of duty; take the high end for poor insulation.
Start-up load
Warming the equipment and pipework metal from cold. For batch or intermittent production this can dominate the selection.
Fluctuation margin
Inlet temperature swings and output ramp-ups — 10–20% margin keeps the heater out of its ceiling in real operation.
04After the kW: what a real specification needs
Power is only the first layer. Then settle: heater architecture (direct or circulation type), design pressure (working pressure plus margin), materials against the medium's corrosivity, orientation and flange standards, control (staged switching or SSR modulation) and protection (over-temperature, low-flow, level).
When you request a quotation, send it complete in one pass: medium name and phase, flow, inlet/outlet temperatures, working pressure, hazardous-area requirements, orientation, control expectations. A complete RFQ skips two rounds of back-and-forth and prices more accurately.